Oil wells vary in depth from a few hundred feet to up to 14,000 feet. Oil is lifted from these depths by a plunger which reciprocates within a pump barrel at the bottom of the well. The plunger is driven by a sucker rod or an interconnected series of sucker rods which extend down from the surface of the oil well to the plunger.
FIG. 1 shows a conventional pump jack 10 for driving the sucker rod of an oil well pump. Pump jack 10 generally comprises a walking beam 12 which is connected through a polished rod 14 to an in-hole sucker rod (not shown). Walking beam 12 is pivotally supported at an intermediate position along its length by a samson post 16, which is in turn mounted to a base frame 18. A drive crank system 20 is also mounted to base frame 18. Base frame 18 is mounted to a concrete base to rigidly locate all components relative to the oil well.
Drive crank system 20 has a rotating eccentric crank arm 24. Crank arm 24 is driven at a constant speed by an electric or gas motor in combination with a gearbox or reducer, generally designated by the reference numeral 26. Eccentric crank arm 24 rotates about a horizontal axis.
Walking beam 12 has a driven end 30 and a working end 32 on either side of its pivotal connection to samson post 16. One or more pitman arms 34 extend from driven end 30 to a crank pin 35 positioned intermediately along outwardly extending eccentric crank arm 24. Rotation of crank arm 24 is translated by pitman arms 34 into vertical oscillation of the walking beam's driven end 30 and corresponding oscillation of working end 32.
Working end 32 of walking beam 12 has an arcuate cable track or horsehead 36. A cable 38 is connected to the top of the cable track 36. Cable 38 extends downwardly along the cable track 36 and is connected at its lower end to polished rod 14. Pivotal oscillation of walking beam 12 thus produces corresponding vertical oscillation of polished rod 14 and of the connected sucker rod. The arcuate shape of cable track 36 ensures that forces between working end 32 and polished rod 14 remain vertically aligned at all positions of walking beam 12.
The sucker rod of an oil well pump performs its work during an upward stroke, when oil is lifted from the well. No pumping is performed during the downward stroke of the sucker rod. Accordingly, a pump jack such as described above supplies force to a sucker rod primarily during its upward stroke. Relatively little force is produced on the downward stroke. To increase efficiency of a drive system counterbalance weights are utilized to store energy during the sucker rod downward stroke and to return that energy to assist in the sucker rod upward stroke.
In pump jack 10, counterbalance weights 40 are positioned at the outermost end of crank arm 24. Such weights could also be positioned on the driven end 30 of walking beam 12. However, a mechanical advantage is obtained by placing the weights outward along the crank arm from the pitman arm connection. During the downstroke of the sucker rod the driving motor must supply energy to raise weights 40 to the top of their stroke. During the sucker rod's upstroke, however, weights 40 assist the motor and gearbox since the outward end of crank arm 24 moves downward while the sucker rod moves upward. The peak energy required by the motor is therefore greatly reduced, allowing a smaller motor to be used with corresponding increases in efficiency.
Mechanical pump jacks such as described above have been used for many years and continue to be used nearly exclusively for driving oil well pumps. Acceptable substitutes have simply been unavailable. One reason for the popularity of such mechanical systems is their extreme simplicity. They do not involve valves, switches, or electronics, and there are a minimum of moving parts. This simplicity results in reliability which is difficult to accomplish with more complex systems. Reliability is of utmost importance since oil well pumps are unattended for long periods, often being located in remote locations.
The very nature of sucker rod displacement created by a reciprocating pump jack is another apparent reason for its success. An oil well sucker rod is often over 14,000 feet long. While reciprocating, it must not only accelerate and decelerate itself, but also a 14,000 foot oil column. In addition, it must accelerate and decelerate oil within an above-surface production line, which can be as long as five miles. Forces caused by sudden acceleration of the sucker rod are therefore very significant. Any such sudden or undue acceleration can stretch and snap the sucker rod.
The pump jack described above minimizes acceleration and deceleration forces on the sucker rod by producing an approximately sinusoidal displacement at the polished rod. The sinusoidal displacement results from translation of rotary crank motion to linear motion at the polished rod. Such sinusoidal motion significantly reduces strain on the driven sucker rod.
However, while the pumping action of a mechanical pump jack is preferable to previously-known alternatives, its physical size creates significant disadvantages. For instance, the great weight of the walking beam, gearbox, and counterbalance weights requires expensive support bases and land site preparation. Rates of reciprocation are often limited by this weight. In addition, pump jacks must be attached permanently above a wellhead and are therefore not easily moved to another site. This results in costly pumping equipment sitting idle during periods of oil well inactivity.
While alternative drive systems have been attempted, none have met with significant commercial success. FIG. 2 illustrates one prior art drive system, comprising a hydraulic pump drive system which is generally designated by the reference numeral 50. Drive system 50 includes a hydraulic cylinder 52 containing a piston assembly 54. Piston assembly 54 is designed for reciprocal vertical motion within cylinder 52. It comprises an elongated center shaft 56 having a pressure piston 58 on its upper end and a working piston 60 at an intermediate position along its length. Center shaft 56 has a lower end which is connected through a coupling 62 to a polished rod 64.
Cylinder 52 has a centrally located annular flange 66 which seals against center shaft 56 between pressure piston 58 and working piston 60 to divide cylinder 52 into an upper pressure chamber 68 and a lower working chamber 70. Pressure piston 58 reciprocates within pressure chamber 68 and working piston 60 reciprocates within working chamber 70.
Piston assembly 54 is driven up and down by hydraulic force applied alternately to the bottom and then the top of working piston 60. A hydraulic pump 72 supplies hydraulic fluid under pressure from a reservoir 74 to a cross-over hydraulic valve 76. Valve 76 is in fluid communication with working chamber 70 through fluid ports both above and below working piston 60. A lower limit switch 78 and an upper limit switch 80 are actuated by a switch actuator 82 which travels up and down with center shaft 56. Actuator 82 actuates lower limit switch 78 at the bottom of desired piston assembly travel, causing cross-over valve 76 to supply pressurized hydraulic fluid to working chamber 70 below working piston 60. This forces piston assembly 54 upward. Actuator 82 actuates upper limit switch 80 at the top of desired piston assembly travel, causing cross-over valve 76 to supply pressurized hydraulic fluid to working chamber 70 above working piston 60. This forces piston assembly 54 back down. Hydraulic fluid displaced by piston 60 from the non-pressurized side of working piston 60 is returned through valve 76 into fluid reservoir 74.
Pressure chamber 68 is filled with hydraulic fluid below pressure piston 58 and is connected for fluid communication with an accumulator cylinder 84. Accumulator cylinder 84 has a free-floating piston 86 which divides accumulator cylinder 84 into a hydraulic fluid chamber 88 and a gas chamber 90. Hydraulic fluid displaced from pressure chamber 68 by the downward ;movement of pressure piston 58 is forced into hydraulic fluid chamber 88, forcing free-floating piston 86 toward gas chamber 90. Gas chamber 90 contains pressurized gas which opposes such movement.
Hydraulic drive system 50 thus provides a hydraulic mechanism for alternately moving a sucker rod upward and downward. Furthermore, the opposing pressure of the pressurized gas within gas chamber 90 assists in the upward stroke of piston assembly 56 and the connected sucker rod. This allows using a smaller hydraulic pump than would otherwise be necessary. The drive system does not, however, address the problems of sudden sucker rod acceleration and deceleration. In fact, the significant force applied to the sucker rod is subject to sudden and complete reversal at both the top and bottom of each sucker rod stroke. The resulting acceleration and deceleration tends to greatly reduce the life of a sucker rod.
Attempts have been made to reduce the sudden acceleration and deceleration which often occurs at the point of stroke reversal in prior art hydraulic pump drive systems. For instance, U.S. Pat. No. 2,555,426 to W. C. Trautman et al. describes using a gas accumulator connected to a hydraulic pressure line which feeds a hydraulic drive cylinder. The gas accumulator is said to maintain a constant pressure on a polished rod so that the velocity of the polished rod can vary according to the resistance encountered and produced by the polished rod and connected sucker rod. However, such an accumulator produces a great degree of elasticity in the drive system, often resulting in uncontrolled and erratic sucker rod displacement. Such uncontrolled displacement itself is a cause of unacceptable acceleration and deceleration. The elasticity in the Trautman drive system prevents it from producing the constant, sinusoidal motion of a pump jack, which experience has proven to be preferable.
The Trautman patent also describes a rather complex valving system intended to modulate the reversal of hydraulic oil pressure to the drive cylinder. Recognizing the desirability of reducing acceleration extremes, Trautman proposes a mechanism for decelerating the drive piston rapidly but uniformly at the end of its stroke, and then accelerating it as rapidly as possible at the beginning of the next stroke (column 9, lines 26-34). Using this approach, full hydraulic pressure is applied at the beginning of each stroke, causing rapid and uncontrolled acceleration of the polished rod and connected sucker rod.
The Trautman mechanism and similar devices have failed to gain any significant acceptance as replacements for mechanical pump jacks. One of the primary disadvantages of such prior art mechanisms is that they involve complex valving systems. Often, the mechanisms require numerous valves, hydraulic pumps, displacement and velocity sensors, and other electronic equipment. Such complexities greatly diminish reliability.
In contrast to the valved mechanisms described above, some prior art systems have used crank-type mechanical drives to reciprocate a master cylinder assembly. U.S. Pat. No. 2,526,388 to William Otto Miller is an example of an oil well pump drive system which uses a mechanically-driven master piston. While drive systems such as described by Miller are significantly simpler than systems utilizing hydraulic switching, they have not been proven to be reliable enough to replace conventional pump drive systems. One significant disadvantage of the Miller system is the driving apparatus used in its master cylinder, shown in FIG. 2 of the Miller patent. The master cylinder utilizes what is known as a "Scotch Crosshead." While this driving arrangement produces a linear displacement thought by Miller to be an improvement over the prior art, it requires a number of sliding surfaces and results in off-center or angularly-misaligned forces which tend to reduce the life of the master cylinder components. The Miller system, perhaps in part because of these reasons, has not been commercially accepted.
The Miller system also does not address the problem of oil leakage in hydraulic systems. Oil leakage can be a significant problem with pumping systems which are installed for continuous and unattended operation for long periods. An automatic method of monitoring and replenishing oil is needed which will not add undue complexity and cost.
The invention described below eliminates virtually all of the complexities of the prior art devices. This results in a hydraulic drive system which emulates the motion of a mechanical pump jack while requiring no valves or variable restrictions during its normal operation. Furthermore, the unique master cylinder mounting arrangement used in the invention eliminates off-center or angularly-misaligned forces at the master cylinder assembly. While providing simplicity in both construction and operation, the preferred embodiment of the invention includes means for automatically regulating pump stroke and for monitoring and automatically replenishing leaked oil. The further advantages of the invention over both mechanical pumping jacks and over prior art hydraulic pump drives will be apparent from the discussion below.